Designing Databases for Multi-Tenant Systems: Shared vs. Isolated Databases

In the rapidly evolving world of software development, businesses are continually seeking efficient ways to scale their applications while managing costs. One popular architecture that supports scalability and resource optimisation is multi-tenancy. Multi-tenancy allows a single instance of a software application to serve multiple customers or "tenants." When building a multi-tenant application, one of the most critical design decisions is how to manage tenant data efficiently and securely. But it raises important questions about how data should be stored and accessed. One of the first decisions to make is whether to use a shared database or a separate database for each tenant. This choice has significant implications for performance, security, and maintainability.

This blog explores how to design databases for multi-tenant systems, highlighting different approaches, the benefits and challenges of each, and best practices for making this crucial decision.

What is Multi-Tenancy Architecture?

Multi-tenancy architecture refers to a system where a single software instance (application, database, etc.) serves multiple distinct customers or tenants. Each tenant's data, configuration, and user experience are isolated to ensure privacy and security, even though they share the same infrastructure.

There are several types of multi-tenancy models:

1. Database-level Multi-Tenancy: Each tenant’s data resides in a separate database or schema.
2. Application-level Multi-Tenancy: Multiple tenants share the same application instance, with tenant-specific configurations and data stored in the same database.
3. Hybrid Multi-Tenancy: A combination of both database-level and application-level isolation, giving more flexibility.

With multi-tenancy, tenants interact with the application in a way that feels personalised to them, but behind the scenes, resources like the database and computing power are shared across many customers. The architecture should ensure that tenants' data remains private, that performance is optimised, and that the system is scalable.

Benefits of Multi-Tenancy Architecture

1. Cost Efficiency: By sharing resources (computing power, storage, etc.) among multiple tenants, businesses can significantly reduce infrastructure and operational costs. It also allows for efficient utilisation of cloud resources, as the costs are split between many users.
2. Scalability: Multi-tenancy enables businesses to scale their applications efficiently. When more tenants are added, the architecture can dynamically allocate resources, ensuring that the system can handle increased demand without a significant increase in overhead.
3. Centralised Maintenance: With all tenants using the same instance, software updates, bug fixes, and security patches can be applied centrally, reducing the complexity of managing multiple instances.
4. Isolation: Despite sharing the same infrastructure, tenants’ data and configurations are kept separate, ensuring that one tenant’s data cannot be accessed by another. This isolation helps maintain security and data integrity.
5. Flexibility and Customisation: Tenants can have customised settings and configurations to suit their unique needs, even within the shared environment.

Challenges of Multi-Tenancy Architecture

1. Data Security and Privacy: Although data isolation is a key feature, multi-tenant systems still face challenges in ensuring that each tenant's data remains secure and inaccessible to others. A breach in one tenant's data could potentially expose other tenants' information if not properly isolated.
2. Performance Issues: Since tenants share the same infrastructure, heavy resource usage by one tenant could affect the performance of others. Ensuring that the system can scale effectively and distribute resources fairly is crucial to avoid bottlenecks.
3. Customisation Limitations: While multi-tenancy offers some level of customisation, there can be limitations to how much each tenant can personalize their experience. Extensive customisation may impact the system’s ability to remain truly multi-tenant.
4. Complexity in Management: As the number of tenants grows, managing their configurations, usage limits, and ensuring the right level of isolation and security can become increasingly complex. Administrators need robust tools to monitor and manage the tenants effectively.
5. Compliance and Regulations: In certain industries, there may be strict compliance and regulatory requirements regarding data storage, processing, and access. Multi-tenancy systems must be designed with these regulations in mind, as some tenants may need stricter data isolation or geographic separation.

How to Consider Database Design for Multi-Tenant Systems

When designing a database for a multi-tenant system, the decision between a shared database and a database per tenant is paramount. Let’s break down the considerations for each approach.

Shared Database Model

In a shared database model, all tenants share the same database instance, but their data is logically separated within the same schema. Each piece of data is tagged with a tenant ID, which allows the application to know which tenant each piece of data belongs to.

GIF

Pros:

1. Simplicity: Managing a single database instance is easier, and schema changes can be made just once. It is a simpler setup for infrastructure management.
2. Cost-Efficient: Using a shared database reduces the overall cost since you aren’t maintaining multiple database instances.

Cons:

1. Risk of Data Leakage: The most significant concern with a shared database is that one tenant's data could accidentally be exposed to another tenant. This risk can be mitigated by carefully including tenant IDs in every query, ensuring all database operations are scoped to the correct tenant.
2. Noisy Neighbour Problem: When one tenant uses a disproportionate amount of resources, it can affect the performance of other tenants. This is particularly problematic if one tenant experiences a sudden spike in traffic. Techniques like rate limiting or database user isolation can help manage this.
3. Scalability Limitations: As the number of tenants grows, scaling the shared database becomes more complex. Performance issues could arise, especially with large datasets or high traffic loads.

To manage the shared database effectively, consider adding a tenant ID column in every table and using filters in your queries to ensure data isolation.

Database Per Tenant Model

In the database per tenant model, each tenant has its own isolated database. This model avoids the risks of data leakage and noisy neighbours because each tenant's data is stored in a completely separate database.

GIF

Pros:

1. Data Isolation: Each tenant’s data is completely isolated, providing an extra layer of security and reducing the chance of accidental data leaks.
2. Performance Isolation: Since tenants have dedicated resources, their usage does not affect others. If one tenant's usage increases, it won’t impact others' performance.
3. Customisation Flexibility: With isolated databases, each tenant can have its own schema, allowing for easier customisation and specific optimisations for individual tenants.

Cons:

1. Management Overhead: Managing a separate database for each tenant can quickly become complex. Schema changes, for example, need to be applied to each database, which can be time-consuming and prone to errors.
2. Higher Costs: Maintaining a database per tenant can increase operational costs, especially if you need to scale horizontally to accommodate many tenants.
3. Complex Migrations: Migrating tenants between databases or consolidating databases for optimisation can be a challenging task, requiring careful planning and execution.

Despite the complexity, this approach is best suited for environments where tenant isolation is a priority or where tenants require highly customised setups.

Hybrid Model: A Combination of Both Approaches

A hybrid model combines the shared and per-tenant database approaches. In this model, some tenants use shared databases, while others use isolated databases. This flexibility allows you to adjust to different tenants' needs and workloads.

GIF

Pros:

1. Flexibility: Allows you to choose the right database model for each tenant based on their requirements. For example, tenants with lower usage or simpler needs may be served by a shared database, while high-demand tenants get their own databases.
2. Scalability: By using a hybrid approach, you can scale efficiently by isolating heavy workloads while still taking advantage of the simplicity of shared databases for smaller tenants.

Cons:

1. Increased Complexity: Managing a hybrid model can be complicated. It requires careful planning to ensure tenants are routed to the right database type and that schema changes are applied appropriately.
2. Migrations: Moving tenants between shared and isolated databases can be difficult, particularly if tenants’ needs change over time.

A hybrid model can be ideal for businesses with a wide range of tenants, some of which need higher performance or stricter data isolation than others.

Considerations for Database Design in Multi-Tenant Systems

When choosing between shared and isolated databases, there are several factors to consider:

1. Number of Tenants: If you have a small number of tenants, a database per tenant might be manageable. However, for larger applications, a shared database or hybrid model might be more feasible.
2. Data Security and Compliance: If tenants' data is subject to stringent security requirements or regulations (e.g., GDPR, HIPAA), a database per tenant may offer better control over isolation.
3. Customisation Needs: Tenants requiring extensive customization in terms of database schema or access rights might benefit from their own database instance.
4. Performance: If performance isolation is a top priority, a database per tenant approach may reduce the risks of one tenant’s usage affecting another. For less demanding workloads, a shared database may suffice.
5. Scalability and Maintenance: As your application grows, consider how you will scale and maintain the database system. A shared database might be easier to manage in terms of infrastructure, but it could require additional strategies to scale as usage increases.

Conclusion

Choosing the right database model for a multi-tenant system is a crucial decision that depends on your application's scale, the level of isolation required, and the complexity of managing tenant data. The shared database model offers simplicity and cost-efficiency, while a database per tenant model provides greater data isolation and performance guarantees. A hybrid model offers a flexible solution, allowing you to tailor your database strategy to the needs of each tenant.

Ultimately, there is no one-size-fits-all answer. Each multi-tenant application has unique requirements, and the best database strategy will depend on multiple business requirements. When implemented correctly, multi-tenancy can be a great solution for businesses looking to deliver software-as-a-service offerings at scale while maintaining robustness, security, flexibility, and efficiency.